1. The Field of the Invention
The present invention relates to embolic protection devices. More particularly, the present invention relates to filters and methods of manufacturing the filters.
2. The Relevant Technology
Human blood vessels often become occluded or blocked by plaque, thrombi, other deposits, or material that reduce the blood carrying capacity of the vessel. Should the blockage occur at a critical place in the circulatory system, serious and permanent injury, and even death, can occur. To prevent this, some form of medical intervention is usually performed when significant occlusion is detected.
Several procedures are now used to open these stenosed or occluded blood vessels in a patient caused by the deposit of plaque or other material on the walls of the blood vessels. Angioplasty, for example, is a widely known procedure wherein an inflatable balloon is introduced into the occluded region. The balloon is inflated, dilating the occlusion, and thereby increasing the intraluminal diameter.
Another procedure is atherectomy. During atherectomy, a catheter is inserted into a narrowed artery to remove the matter occluding or narrowing the artery, i.e., fatty material. The catheter includes a rotating blade or cutter disposed in the tip thereof. Also located at the tip are an aperture and a balloon disposed on the opposite side of the catheter tip from the aperture. As the tip is placed in close proximity to the fatty material, the balloon is inflated to force the aperture into contact with the fatty material. When the blade is rotated, portions of the fatty material are shaved off and retained within the interior lumen of the catheter. This process is repeated until a sufficient amount of fatty material is removed and substantially normal blood flow is resumed.
In another procedure, stenosis within arteries and other blood vessels is treated by permanently or temporarily introducing a stent into the stenosed region to open the lumen of the vessel. The stent typically includes a substantially cylindrical tube or mesh sleeve made from such materials as stainless steel or nitinol. The design of the material permits the diameter of the stent to be radially expanded, while still providing sufficient rigidity such that the stent maintains its shape once it has been enlarged to a desired size.
Unfortunately, such percutaneous interventional procedures, i.e., angioplasty, atherectomy, and stenting, often dislodge material from the vessel walls. This dislodged material can enter the bloodstream, and may be large enough to occlude smaller downstream vessels, potentially blocking blood flow to tissue. The resulting ischemia poses a serious threat to the health or life of a patient if the blockage occurs in critical tissue, such as the heart, lungs, kidneys, or brain, resulting in a stroke or infarction.
Some existing devices and technology use a filter for capturing the dislodged material from the bloodstream. Usually, the filter includes a filter material which includes pores or openings to allow the blood to pass therethrough while at the same time preventing larger debris from passing therethrough. Generally, the filter material is constructed from an organic or inorganic polymer. Suitable polymeric materials can be formed into a thin film using a variety of techniques such as extrusion, dip molding, stretching, casting, and calendering.
Polymer films can be constructed quite thinly, usually having wall thicknesses of about 1 mil (25 μm). However, it would be an advantage to have a polymer film that has an even thinner wall thickness. Having a thinner film or material allows a greater volume of filter material to be packed into a mechanism to deploy the filter. As a result, more filtering media is provided for the filtering function, allowing more debris to be caught by the filtering mechanism.
In addition, a desired characteristic of filter material is that it has a degree of elasticity or some elastomeric attribute. As the fluid is passing through the filter material, it is desirable that the filter material is somewhat elastomeric so that the flow of the bloodstream does not rupture the filter material.
The filter material should also have high tear strength to withstand the fluid flow across the surface area of the filter material. In addition, it is desirable that the filter material have high tensile strength for many of the same reasons that it should have high tear strength and have elasticity.
In order to maintain the desired strength and flexibility of the filter material, it is important that the filter material maintain a consistent thickness. In many types of thin materials, generally as the thickness of the material decreases, these desired qualities of high elasticity, high tear strength and high tensile strength also decreases. Thus, certain processes, such as dip coating, are not suitable for providing filter material with consistent thickness. In the dip coating process, an operator dips a mandrel into polymer which is dissolved in a carrier or solvent. The mandrel is then withdrawn at a certain velocity. Depending on the rate of withdrawal and depending on the viscosity of the fluid, the thickness of the filter material is controlled. However, when the solvent comes off the mandrel, it results in an inconsistent thickness of the filter material. In addition, the dip coating process is limited on how thin the filter material can be formed. Furthermore, the dip molding process at the molecular level results in a very homogeneous molecular structure that actually reduces the tear strength and tensile strength of the resultant filter material.
Where the filter material is a thin film, the thin film requires special handling. As such, manufacturing costs can be quite high due to the delicate and thin nature of the material. However, it would be desirable to make the manufacturing process steps as simple as possible to reduce costs. In addition, it is desirable to make a process which is safe. For example, PTFE (polytetrafluoroethylene) film can be made from extrusion or stretching processes. However, PTFE films are generally difficult to work with, requiring dangerous solvents or heat bonding processes. Furthermore, extrusion or stretching processes still do not produce the desired film thinness or the desired tear strength required for bloodstream filtering applications.
In addition, it is desirable to be able to pack the filter material within some sort of deployment mechanism in order to navigate the filter material through the vascular system of the patient. Once a filter mechanism is positioned at the correct location, the filter material should then be easily deployed such that it closely reforms to its original shape. Various characteristics of the filter material influence its ability to be restored back to its original shape. Such characteristics include compliancy, stiffness, thickness, resiliency, and elastomericity.
Many conventional filter materials do not have adequate compliancy, are too stiff, are too thick, not sufficiently resilient or elastomeric to reform back to the original shape when deployed. Rather, they usually remain partially compressed or end up having many folds or wrinkles on the surface area thereof. However, such folds and wrinkles produces what is known as hysteresis. Existence of hysteresis in the filter material has a potential to weaken, or even rupture or tear the filter material at the location of the fold or wrinkle. Additionally, folded areas may not be fully exposed to the blood flow so as to decrease the efficiency of the filtering. Thus, it is advantageous to reduce the amount of hysteresis in the deployed filter media.